In recent years, radio cellular systems represented by cellular phone sets or the like have diversified service modes and are required to transmit not only speech data but also large amounts of data such as still images and moving images.
While services of IMT-2000 cellular systems are already in progress, 3GPP LTE (3GPP Long Term Evolution) or the like requiring a peak rate of 100 Mbps in downlink is also being standardized. Now, standardization of IMT-Advanced is about to start as a category aiming for further evolution. This IMT-Advanced manifests requirements of about several Gbps as a rate with 100 MHz bandwidth in downlink and about several Mbps with 40 MHz in uplink, and requires a breakthrough far beyond IMT-2000.
Especially, frequency resource allocation or link adaptation matching frequency responses (i.e. quality/CQI (Channel Quality Indicator) for each frequency segmented within a band) is indispensable to make bands wider in both uplink and downlink and effectively use radio resources in such a circumstance. However, in case of FDD (Frequency Division Duplex) using different frequencies between uplink and downlink, frequency responses differ between an uplink band and a downlink band, and therefore both a radio base station apparatus (i.e. Node B) and mobile terminal apparatus (i.e. UE) need to transmit pilot signals or the like to measure a frequency response of each band.
Although transmitting data usually requires pilot signals for synchronized detection, the pilot signals for such a purpose only need to be transmitted in a band in which data is transmitted. However, pilot signals for quality measurement required for frequency resource allocation need to be transmitted in an entire band or in a band wider than the band in which data is transmitted, regardless of whether or not there is data.
On the other hand, in case of TDD (Time Division Duplex) which uses the same frequency in uplink and downlink, uplink and downlink have a high correlation in the channel characteristics (i.e. channel reciprocity). Therefore, a mobile terminal apparatus (i.e. UE) measures a frequency response (to be more specific, an SINR (Signal-to-Interference plus Noise Ratio) for each frequency band) using pilots transmitted by a radio base station apparatus (i.e. Node B) in downlink, the mobile terminal apparatus reports this frequency response (to be more specific, CQI (Channel Quality Indicator) for each frequency band) to the radio base station apparatus, so that the radio base station apparatus can learn frequency responses not only in downlink but also in uplink (see Patent Document 1 and Non-Patent Document 1). Therefore, the mobile terminal apparatus need not send pilot signals to measure quality in an entire band or wide band, to the radio base station apparatus. Therefore, TDD can reduce power consumption of the mobile terminal apparatus compared to FDD.
Furthermore, as described above, IMT-Advanced requires a wide band to realize high-speed transmission. Therefore, compared to IMT-2000, cases might occur in IMT-Advanced where widebands cannot be used in uplink and downlink individually depending on regions and circumstances of the operator. That is, when bands are made wider in both uplink and downlink, the importance of the TDD scheme which uses the same frequency in uplink and downlink is expected to increase more than ever.
Patent Document 1: Japanese Patent Application Laid-Open No. HEI8-223106
Non-Patent Document 1: “A Study on Reduction of CSI feedback for Frequency Domain Scheduling in OFDMA/TDD Systems”, Haruka OBATA, Tadashi BABA, Seiich SAMBE, TECHNICAL REPORT OF IEICE, RCS2006-5, April 2006.